This section provides background information related to the present disclosure that is not necessarily prior art. Thermoelectric materials may be used for direct conversion of heat to electricity and, thus, can substantially increase the efficiency of energetic processes. Current state of the art thermoelectric materials are comprised of elements which are in low abundance and often toxic.
In the past few decades, thermoelectric (TE) materials have been a focus topic in solid-state physics and materials science due to their potential application in waste energy harvesting or Peltier cooling. The efficiency of thermoelectric materials is evaluated by the figure of merit (ZT=S2σT/κ), where S is the Seebeck coefficient, a the electric conductivity, T the absolute temperature, and κ thermal conductivity. For many years, the benchmark for a good thermoelectric material has been ZT of order unity, typified by Bi2Te3 and its alloys which are used commercially in thermoelectric cooling modules. Over the last 15 years, with a more complete understanding of electronic and thermal transport in semiconductors, better control over synthesis methods, and the successful application of nanotechnology, new materials systems with ZT values higher than unity have been discovered and developed, including thin film superlattices, filled skutterudites, and bulk nanostructured chalcogenides. Unfortunately, many of these new materials are not suitable for large scale application because of complex and costly synthesis procedures, or the use of rare or toxic elements.
A current challenge is the discovery of new thermoelectric materials which are inexpensive, environmental-friendly, easy to synthesize, and comprised of earth-abundant elements. The old concept of the Seebeck effect, which describes how heat can be converted into electricity in thermoelectric (TE) materials, has been an active area of research in solid state physics and materials science in the past two decades. The performance of a thermoelectric material is characterized by the dimensionless figure of merit zT=sfffT/K, where S is the Seebeck coefficient, a the electrical conductivity, T the absolute temperature, and κ thermal conductivity. For traditional thermoelectric materials, zT values are typically of order of unity. Higher performance thermoelectric materials can be realized by improving the power factor (S2{circumflex over ( )}) or reducing thermal conductivity. By band structure engineering, tuning the carrier concentration, or introducing nanostructures, zT values can be raised to more than 1.5 or even higher at high temperature, as has been shown for some filled skutterudites and bulk nanostructured chalcogenides. Very recently, Biswas et al. reported that PbTe—SrTe doped with Na shows a maximum zT value of 2.2 at 923K due to a hierarchical structure that maximizes phonon scattering. Unfortunately, however, many of these new materials use rare or toxic elements, impeding their application on a large scale. Some work has been done to avoid these rare or toxic elements; for example, Ca3AlSb3 with zT value of 0.8 at 1050 K has been reported,4 and PbS nanostructured with SrS and CaS shows a zT value of 1.2 at 923K.5 Nevertheless, these systems still require complex and carefully controlled synthesis procedures.
One very successful route to improving ZT in bulk solids is reduction of lattice thermal conductivity. For instance, the notion of “phonon glass/electron crystal (PGEC)” was introduced to describe materials that exhibit lattice thermal conductivity like a glassy or amorphous solid, and electronic properties of a good crystal. For amorphous or glassy solids, the phonon mean free path approaches one interatomic spacing; a phonon mean free path shorter than one interatomic spacing loses its meaning, and thus this type of thermal transport has been termed “minimal” thermal conductivity. Unfortunately, poor electrical conductivity in such amorphous solids prevents them from exhibiting high values of figure of merit. More interesting from the thermoelectric point of view are crystalline solids which exhibit minimal thermal conductivity, due to strong intrinsic phonon scattering. Examples here include, in addition to the afore-mentioned skutterudites, complex cage structures such as clathrates. Recently, minimal thermal conductivity was discovered in crystalline rocksalt structure I-V-VI2 compounds (e.g., AgSbTe2), semiconductors typified by the lattice thermal conductivity of a glassy or amorphous system. These materials exhibit electronic properties characteristic of good crystals and thus have demonstrated good thermoelectric behavior.
Recently, a correlation between minimal thermal conductivity and the existence of an Sb lone pair in Sb-containing ternary semiconductors has been identified. Lone pair electrons induce large lattice anharmonicity that gives rise to thermal resistance. Using density functional theory calculations, it has been demonstrated explicitly the occurrence of large Grüneisen parameter in Cu3SbSe3 compounds and, using these parameters to calculate phonon scattering rates, were able to quantitatively account for the thermal conductivity using the Debye-Callaway formalism.
One further issue with the formation of these materials comes from the general chemistry of the problem. In addition to stoichiometric precision, crystal structure is also very important. In this regard, when more than three materials are mixed, heated, and then cooled, many of the materials will preferentially combine into crystal structures which do not possess the preferred crystal structure to maximize thermoelectric effects. In this regard, phases such as Cu3SbS4 that have high thermocoefficients can be formed, thus reducing the thermoelectric properties of these materials. While bulk heating of these materials can convert some of the crystal structures to desirable forms, the net result is far from assured.
According to the present teachings, a thermoelectric device is provided. The thermoelectric device has a pair of conductors and a p-type thermoelectric material disposed between the conductors. The thermoelectric material is at least partially formed of a hot pressed high energy milled tetrahedrite formed of tetrahedrite ore and pure elements to form a tetrahedrite powder of Cu12-xMxSb4S13 disposed between the conductors, where M is at least one of Zn and Fe.
According to the present teachings, a method of producing a thermoelectric device is provided. The method included high energy milling tetrahedrite having natural tetrahedrite ore and pure elements to form a tetrahedrite powder of Cu12-xMxSb4S13 wherein M is selected from the group of Zn at a concentration 0<x<2.0, Fe at a concentration 0<x<1.5, and combinations thereof. The high energy milled tetrahedrite is hot pressed to form a pellet to a density greater than 95%. The pellet is then disposed between a pair of electrical conductors.
According to the present teachings, a thermoelectric is performed. The material is formed of a high energy milled tetrahedrite comprising natural tetrahedrite ore and powder elements to form a tetrahedrite powder of Cu12-xMxSb4-yAsyS13, wherein M is selected from the group of Zn at a concentration 0<x<2.0, Fe at a concentration 0<x<1.5, and combinations thereof.
According to the present teachings, a thermoelectric device provides a pair of thermal conductor, and hot pressed high energy milled tetrahedrite comprising natural tetrahedrite and powder elements milled to form a tetrahedrite powder of Cu12-xMxSb4S13 disposed between the thermal conductors, where M is one of Zn and Fe.
According to the present teachings, a thermoelectric device provides a pair of conductors and a p-type thermoelectric material disposed between the conductors. The thermoelectric material is formed of hot pressed high energy milled tetrahedrite formed of tetrahedrite ore and pure elements to form a tetrahedrite powder of Cu12-xMxSb4S13 disposed between the conductors, where M is at least one of Zn and Fe.
According to the present teachings, a method of producing a thermoelectric device is provided. The method included high energy milling tetrahedrite comprising natural tetrahedrite ore and pure elements to form a tetrahedrite powder of Cu12-xMxSb4S13 wherein M is selected from the group of Zn at a concentration 0<x<2.0, Fe at a concentration 0<x<1.5, and combinations thereof. The tetrahedrite is hot pressed to form a pellet to a density greater than 95%. Lastly, the pellet is disposed between a pair of electrical conductors.
The chemical compositions described herein are synthesized from earth abundant materials and in some cases can be extracted in nearly ready-to-use form from the earth's crust. Furthermore, the compounds are comprised of elements of low atomic mass, such that the density of the compounds is significantly less than state of the art compounds. These compounds can be used in provide, lightweight, low-cost thermoelectric devices for large scale conversion of heat to electricity.
Cu12Sb4S13, the base composition of a large family of natural minerals called tetrahedrites, is structurally very closely related to the Cu3SbSe3 phase; its unit cell can be considered as quadruplicate of the Cu3SbS3 unit cell. It possesses a cubic sphalerite-like structure with six of the twelve Cu atoms occupying trigonal planar sites with the remaining Cu atoms distributed on tetrahedral sites. In terms of a crystal-chemical formula, four of the six tetrahedral sites are thought to be occupied by monovalent Cu, while the other two are occupied by Cu2+ ions; the trigonal planar sites are occupied solely by monovalent Cu. Magnetic measurements supporting the present invention reveal that antiferromagnetic interactions occur between the Cu2+ ions and induce a magnetic ordering transition below 83 K. The Sb atoms also occupy a tetrahedral site but are bonded to only three sulfur atoms, leading to a void in the structure and a lone pair of electrons, just as Cu3SbSe3. A powder processing procedure is disclosed using natural mineral tetrahedrite ore, the most widespread sulfosalt on earth to provide a low cost, high throughput mechanism of producing thermoelectric materials with high conversion efficiency.
The current teachings are superior to the prior art because they describe compounds that 1) are made from earth-abundant elements and are themselves common and widespread minerals in the earth crust; 2) consist of elements of light atomic mass, leading to low density and ultimately lower weight devices; 3) require no special processing beyond melting, annealing, and powder processing; 4) exhibit large thermoelectric figure of merit can be maintained over a wide range of composition, simplifying the synthesis procedure; and 5) are of composition that span the range of compositions of the large mineral families of tetrahedrite and tennantite, indicating that these minerals may be used directly as source materials for high efficiency thermoelectrics, leading to considerable cost savings.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.